It took me a while to review this book, as its topic is quite specific and I didn't know enough of the technologies to make sense of it. So, in the meantime, I got a couple of books on Ruby and Rails, and then continued the review.
Google have published the API for their web-based maps technology, allowing you create your own web sites/applications. This book takes you through the process of creating the "Hello World" Google maps application, adding interaction via Ajax, and goes on to cover some advanced mapping concepts, which you may need to produce a heavily-used site, or to provide your own datasources. Geocoding is covered, using a number of sources including Google's own and Yahoo's.
Source code is provided on line, although it didn't always match what was in the book, and it wasn't clear which stages of the chapter the various folders referred too, but it did build and run.
Generally, I found the book competent, and described the concepts well. The different chapters sometimes build on the previous ones, sometime start something new, so you have to pay attention to what the code is and where it is supposed to go.